Activation of antilipolytic alpha(2)-adrenergic receptors by epinephrine during exercise in human adipose tissue

The involvement of the antilipolytic alpha(2)-adrenergic pathway and the sp ecific role of epinephrine in the control of lipolysis during exercise in a dipose tissue (AT) were investigated in healthy male subjects (age: 24.1 +/ - 2.2 yr; body mass index: 23.0 +/- 1.6). An in vitro study carried out on isolated adipocytes showed that the weak lipolytic effect of epinephrine wa s potentiated after blockade of alpha(2)-adrenergic receptor (AR) by an alp ha(2)-AR antagonist and reached that of isoproterenol, a beta-AR agonist. T he effect of the nonselective alpha(2)-AR antagonist phentolamine on the re sponse of the extracellular glycerol concentration (EGC) in AT during two s uccessive bouts of aerobic exercise (50% maximum O-2 uptake, 60 min duratio n) was evaluated using the microdialysis method. The metabolic responses me asured in perfused probes with Ringer solution were compared with those obt ained in perfused probes with Ringer plus 0.1 mmol/l phentolamine. Plasma n orepinephrine level was not different during the two exercise bouts, wherea s that of epinephrine was 2.5-fold higher during the second exercise. EGC i n AT was twofold higher in the second compared with the first exercise, and the same response pattern was found for plasma glycerol. The exercise-indu ced increase in EGC was higher in the probe perfused with phentolamine comp ared with the control probe in both bouts of exercise. However, the potenti ating effect of phentolamine on EGC was significant during the second exerc ise bout but did not reach a significant level during the first. These resu lts suggest that epinephrine is involved in the control of lipid mobilizati on through activation of antilipolytic cut-AR in human subcutaneous AT duri ng exercise.